It's all gone a bit wrong with Starrett.
They shipped me a replacement directly from the US but not only was it the wrong graduation (metric only instead of mixed), it has the same issue I reported to them in the first place.
Pretty alarming this slipped past their QA (if they even have a QA anymore) and yeah, colour me unhappy.
I thought the problem had developed over time but seeing as a brand new one (fresh from factory as I understood from their comments) has the same issue, it was probably there from the very beginning.
Kicking myself in hindsight for not doing a closer inspection after I first bought it, but it came quite greased up and I just did a cursory wipe before using it. Thought this was going to be a problem-free purchase from (supposedly) the best!